SSL-mode error reporting in libpq

In testing the fix for the SSL problem that Martin Pihlak reported, I
realized that libpq doesn't really cope very well with errors reported
by OpenSSL. In the case at hand, SSL_write returns an SSL_ERROR_SSL
code, which pqsecure_write quite reasonably handles by putting
"SSL error: bad write retry" into conn->errorMessage. However, it
then sets errno = ECONNRESET, which causes its caller pqSendSome()
to overwrite that potentially-useful message with an outright lie:
"server closed the connection unexpectedly".

I think what we ought to do is adjust the code so that in SSL mode,
pqsecure_write is responsible for constructing all error messages and
pqSendSome should just leave conn->errorMessage alone.

We could perhaps go a bit further and make pqsecure_write responsible
for the error message in non-SSL mode too, but it looks to me like
pqSendSome has to have a switch on the errno anyway to decide whether to
keep trying or not, so moving that responsibility would just lead to
duplicative coding.

Any objections?

regards, tom lane